I use the Wild Oasis floorless tent by Six Moon Designs. It weighs less than one pound and has plenty of room for one plus gear. If you need a tent floor the Lunar Solo is the same thing but with the floor. I own both, but like the weight savings of the Wild Oasis. The owner of Six Moon Designs is awesome. I highly recommend it. Like what others have said, bivy's are only comfortable when you don't need to be in it.
http://www.sixmoondesigns.com/shop/shopexd.asp?id=48